Poweing GPU with 8 pin + 6 pin leading to single 8 pin

[Driver92333]

I have a gtx 970 g1 gaming GPU. It requires an 6 pin and an 8 pin (6 + 2 pin) for power. My new power supply, an EVGA 650 watt p2 power supply comes with GPU power cables that have one 8 pin (6 + 2) and a 6 pin leading to a single 8 pin that goes into the PSU. Can I get away with using one cord coming from the PSU to power the card or should I run a second cable for the 6 pin. I'm concerned about the wattage/volts going through the single 8 pin connector at the PSU.

 

[Karadjgne]

Use the intended cables. If your psu is setup for a single 8pin at psu and a split 6+2 / 6 pin at the gpu then that's exactly what you need. Corsair is doing the same thing with the CXM 450W, 1 lead, 2 connectors.

 

[JackNaylorPE]

Ah... the EVGA "stoopid cables" i call them:

8 pin connector rated at 150 watts
6 + 2 rated at 150 watts
6 rated at 75 watts.

So if ya use the 8 pin with the 6 + 2 pins at the other end, you fine
So if ya use the 8 pin with two 6 pins at the other end, you fine

You use the 8 pin with a 6+2 (150 watts) and the 6 (75 watts) and you exceed the 150 watt rating of the 8 pin connector. Do NOT use in this manner.

 

[Karadjgne]

No. An 8pin pcie is rated at 150w, a 6pin pcie is rated at 75w. The 8pin psu side has no such number, it's dependent on the psu itself and the wire guage of the lead. Could just as easily have run a 4pin at the psu or a 10pin. A single, foot long 18ga wire at 105°rating can carry upto 18A at 12v DC (192w), so a single lead with 2 hots can easily carry over 380w, well within specs of 150+75.

A single lead with 6+2pin/6pin terminations is fine as long as the lead has 2x hots. Generally, those psu side 8pins are 4x hot, 4x ground, unlike the pcie which are 2x/4x and 3x/5x hot/ground.

And to be honest, that 75/150 is not really accurate. The pins used in those pcie molex connections are rated at 5A per pin. On a normal 6pin pcie, that's actually 120w, on the 8pin it's 180w. 75/150 was adopted for universal safety issues due to heat output because full, solid connections can't be guaranteed with spreadable pins subject to stresses and wear. With a 4x/4x pin layout, an 8pin psu side connector has at least the same wiring capability as 4x 6pin pcie.

 

[JackNaylorPE]

1. The same wires are at both ends of the cable.

2. The same connector components are at both ends of the cable.

3. It's like having rigging assembly that picks up a a large load. A chain is no stronger at one end than it is at the other. When we install underground concrete structures, they are lifted with a rigging chain which connected to a ring that has 3 chains. All chains are the same size and same rating ... if the rigging apparatus chains are rated say at 10,000 pounds each, the three chains below the ring do not increase the capacity of the single chain from the ring to the machine doing the lifting. You can likely lift 5 times the "rated load" before that chain breaks ... but don't do it with a safety inspector on site. Those chains are subject to dings, notches, corrosion , repetitive load cycling all of which decreases strength over time and relying on break strength will eventually get someone killed.

So the issue is rating versus ultimate capacity. And the rating is below capacity for a reason:

1. All but the cheapest PSUs can exceed their rating .... but it's not recommended

2. Intel CPUs can go up to just under 100C .... but it's not recommended

3. nVidia GPUs have max temps in the neighborhood of 95C .... but it's not recommended... nVidia starts throttling at 82C

4. Every cable, outlet, switch and electrical device known to man has a rating below it's ultimate capacity. When designing power service to a building, if I was to ignore ratings and design based upon failure points, I'd be in violation of all applicable codes and I would lose my license to practice.

So yes, ratings are not intended to convey the maxium capacity but, as you said ... "for universal safety issues due to heat output because full, solid connections can't be guaranteed with spreadable pins subject to stresses and wear"

I have a Seasonic cable like that ... except it has more than 8 cables/connector points at the PSU end.

 

[Karadjgne]

Jack, count the pin settings. Pcie 6+2pin has just 3x hot wires, 3x solid ground wires, and 2x ground/sensor wires. The psu side of that connector has 4x hot pins and 4x ground pins. At a max of 60w per pin, that's a max of 60x4 (240) per psu side, while gpu usage of the gpu side will show limits of (supposedly) 75+150 (225). For any modern gpu, that's plenty. But granted, older gen amd gpus like the 290's pulling spikes of over 290w on 8+6+pcie (300w) might give pause for consideration. For a 150-170w gtx970? Drop in the bucket, no worries